There exist various techniques for measuring and accurately positioning laser beams. The following early publications represent the general state of the prior art:
"Laser Recording Performance With Spatial Error Compensation", S. Bousky and L. Teeple, SPIE, Vol 53, 1973. In this paper, a polygon deflector is employed together with a reference line for determining cross scan error. Two-dimensional corrections are made to the beam position, which is measured differentially.
"Beam Deflection at High Accuracy and Precision", D. P. Jablonowski and J. Raamot, SPIE, Vol 84, 1976. This paper describes X-Y deflection using galvanometer-driven mirrors or polygons, using crossed gratings to measure two-dimensional position information. A reference beam is deflected to measure position and is used to measure the beam position in two dimensions during writing.
"Ultrahigh Resolution Data Terminal", M. R. Smith, R. H. Burns and R. C. Tsai, SPIE, Vol. 200, 1979. This paper describes a high resolution display that uses a laser to address a liquid crystal gate. Deflection is performed by a galvanometer-driven mirror. A referece beam hits a crossed Ronchi grating and the resulting information is used to instantaneously determine the two-dimensional coordinates of the beam.
One of the main problems encountered in high-quality laser beam scanning relates to accurate positioning of a laser generated dot relative to adjacent dots, as noted in the literature (e.g. Bestenheimer et al. in Journal of Appl. Phot. Eng. vol. 2, 1976). The human eye has extreme sensitivity to periodic errors in the interline separation of beams. Periodic errors resulting from intensity changes, or positional fluctuations, can lead to banding of high visibility. Passive and active methods have been used to minimize these errors.
One of the applications of laser beam scanners is readout of information from previously recorded data. Since the illuminating laser beam is usually extremely directional, light must be reflected from as wide an angle as possible, to minimize speckle noise, increase signal-to-noise ratio and eliminate visibility of cut-lines in graphic arts paste-ups. It is a purpose of this invention to offer solutions to these two problems.